1. Technical Field
This invention relates generally to optical inspection instruments and, more specifically, to such instruments that inspect a surface of interest, such as a wafer, for example during in-line processing.
2. Discussion of the Related Art
There has been much investigation in the field of inspection systems. In particular, inspection systems are needed in the semiconductor industry for inspecting wafers. The significance for detecting and identifying submicron surface defects on such wafers is due in part to the present semiconductor industry move from 0.250 micron to 0.180 micron and, in less than ten years, to 0.130 micron fabrication architectures. The latter architecture is so exacting that it will require the detection of 2 nm substrate defects and 40 nm sized particles on unpatterned silicon wafers. In addition, the industry is scaling up from 200 mm to 300 mm diameter wafers with fewer defects commercially permitted, and rapid detection required at all processing stages. The current industry projection is that about 9% of all wafer production will use the 300 mm format by the year 2002. To meet these needs, defect data must be processed in near-real-time to allow correction of processing problems through, for example, statistical process control (SPC) techniques.
Many surface roughness inspection systems are available. These include: high resolution microscopes such as the atomic force microscope, optical microscopes such as the phase contrast microscope, other optical measurement systems such as ellipsometers, and mechanical contact methods that use a stylus. For sub-micron resolution, most of these techniques are not suitable for in-process surface inspection. High-resolution microscopes require cumbersome surface preparations and expensive operations. Optical microscopes, in general, do not have sufficient resolution and accuracy. Ellipsometry or spectroscopy also do not provide adequate surface roughness information. Mechanical stylus devices are simply out of the question.
Among the possible methods is the optical heterodyne (frequency-shifted) microscope. The heterodyne microscope is an interferometric microscope where the signal beam is frequency-shifted relative to a reference beam. With this method, the signal containing the optical phase information (surface roughness) can be electronically detected by comparing the phase of the beams from different portions of the wafer surface. The problems with this method are: (a) a critical focusing requirement, (b) low throughput rate due to slow scanning, (c) inadequate lateral resolution ( greater than 0.2 mm), (d) limited sensitivity ( less than 100 nm), and (e) inadequate information on the surface deposit materials. In particular with regard to problem (b), a serious drawback with microscopy, in general, is in assessing the number of defects over a large wafer area by scanning with a micron-sized area of view. This would require hours, if not days, to view an entire wafer surface even with a multiple array of detectors.
Another candidate technology, namely scatterometry, has a drawback as presently, generally implemented. A significant limitation with scatterometry is that the intensity of scattering is typically measured at oblique angles, excluding the specular beam. Under this condition, the diameter of particles that can be realistically detected using an in-line tool must be greater than 80 nm (i.e., in order to capture enough light to detect particles, the particle size must be greater than about 80 nm.
U.S. Pat. No. 5,343,290 to Batchelder, et al describes a dark field illumination heterodyne interferometer for particle detection. A heterodyne interferometer is combined with a dark field illumination for improved surface particle detection sensitivity. The U.S. Pat. No. 5,486,919 to Tsuji also describes an optical heterodyne interferometer to detect defects on patterned wafers utilizing different states of polarization for incident and scattered light. The Batchelder and Tsuji heterodyne interferometers provide photon counting detection (Shot noise limited) for the small scattered light from the particles, although the grazing illumination described in both patents will only provide lower sensitivity (only good for a larger particle detection). The problem with these methods is that they cannot effectively discriminate particles on the wafer surface, (which are desired to be detected) from particles within the beam suspended in the air, (which are not desired to be detected.)
U.S. Pat. No. 5,030,842 to Koshinaka, et al describes a method to detect particles using two illumination beams, where the phase of one beam with respect to the other is modulated so that the detected light scattered from a particle in the illuminated area can be distinguished from all other light. As they point out in their patent, however, it will detect particles suspended in the air as long as they are within the overlapped region of the two illumination beams. This will increase a chance of producing a false signal.
U.S. Pat. No. 6,081,325 to Leslie, et al describes a sophisticated scatterometer, which can detect defects and particles on unpatterned and patterned wafer surfaces. The system uses several photo-multiplier tubes and a charge-coupled detector (CCD) camera. A focused laser beam illuminates the sample surface and the light scattered from a particle or defect is monitored by these multiple numbers (4) of photo-multiplier tubes. Utilizing the asymmetrical nature of scattering from particles or defects, the system discriminates between defects (or particles) and the wafer pattern. In addition they use a CCD camera and obtain an image of the sample surface illuminated by the focused laser beam: light scattered from only the illuminated spot is focused on a pixel of CCD camera. The wafer is scanned multiple times to provide a desirable image. However, this imaging method does not provide photon-counting detection. Also, the rescanning of the surface is time consuming a particularly undesirable for commercial applications.
The objective of the present invention is to overcome these shortcomings of the existing devices and to achieve photon detection using a CCD camera, and to completely discriminate between particles on a sample surface and those suspended in air. This is achieved without using any external frequency shift or phase modulation device and focused illumination beam scanning.
In accordance with the present invention, an apparatus is provided for inspecting a surface of an object. The apparatus includes the following major parts: a light source, a photodetector, an optical assembly, a positioning assembly, and a controller. The light source is configured to generate an illumination light beam. The illumination beam, when incident upon a particle on the surface, is scattered to define a scattered beam. The optical assembly is configured to direct the scattered beam and a reference beam, derived from the illumination beam, to the photodetector. The positioning assembly is configured to move the object such that the surface moves relative to the illumination beam. Finally, the controller is coupled to the photodetector and is configured to detect the presence of the particle in accordance with an interference pattern from a superposition of the reference beam and the scattered beam.
The invention is suitably adapted for use in the inspection of semiconductor wafers, using optical detection to holographically record light scattered from a particle on a wafer surface illuminated with, in a preferred embodiment, a laser or other coherent source. The invention utilizes two beams, at least one of which is incident on a wafer surface. These two beams are preferably derived from the same source and are subsequently re-combined to form the above-mentioned interferometric pattern. In one embodiment, a geometrical configuration is used such that the illumination beam strikes the surface at normal or near normal incidence. A specular reflection of the illumination beam from the wafer surface is used as the reference beam and is interferometrically combined with light backscattered (the xe2x80x9cscattered beamxe2x80x9d) from any contaminant particles on the surface. The device, when used in a preferred environment, is especially effective for scanning unpatterned semiconductor wafers, since they have only a relatively few, isolated contaminant particles present.
In a preferred embodiment, the light source may comprise conventional apparatus, such as a diode-pumped solid-state laser, a multiple wavelength ion laser, or even a partially coherent source like a xe2x80x9cwhite-lightxe2x80x9d Xenon lamp. In operation, a wafer surface is illuminated by the illumination beam. The scattered light from a deposit (particle) is detected as a fringe pattern formed by the interference between the scattered light and a reference beam generated from the same light source. The interference pattern will be a spatially fluctuating component of the light power, which can be detected using the photodetector.
In another preferred embodiment, the interference pattern is preserved during scanning. This is accomplished by using Time Delay and Integration (TDI). In particular, the preservation is accomplished by transporting the wafer surface such that the modulated light signal (i.e., interference pattern) moves across the face of the photodetector in synchronism with an electronic position shift of the register portions of the photodetector. Since the shifting of the image data in the registers is substantially locked to the scanning speed of the wafer, the inspection apparatus according to the invention is basically immune against background optical noise, such as that caused by scattering particles in the air. In the described embodiment, the apparatus may operate as a xe2x80x9cphoton-counterxe2x80x9d, limited only by the quantum effect shot noise.
In yet another embodiment, composition analysis of detected particles is performed. The composition analysis of such a contaminant particle is carried out using a spectrometer. By analyzing the spectrum of the interference pattern using predetermined data, the composition of the contaminant can be identified.
Inasmuch as the amplitude of the scattered signal is proportional to the volume of the contaminant particle, the controller can determine volume density. The apparatus detects the scattered beam as a modulation pattern caused by the interference between the reference beam and the scattered beam. This interference pattern can be detected with, in a preferred embodiment, a CCD camera.
In such embodiments employing a CCD camera, the CCD camera may be configured in a variety of ways including, but not limited to, both a linear scanning array and an area array. This latter configuration can function as an integrating device or as a frame transfer camera. As an integrating device, the CCD camera may be controlled by the controller to operate as a TDI sensor. In this technique, the detector array registers are clocked so that the charge packets are transferred in a manner synchronous to the movement of the wafer by the positioning assembly. Image data analysis may be conducted using conventional digital signal processing methods.
With this interferometric detection apparatus, weak scattering from particles in the range of 20 nm to 100 nm in size may be detected. This compares to conventional scatterometry which can, at best, detect 80 nm or larger particles. The amplitude of the scattered light is also dependent on the refractive index of the particle. Since, in one embodiment, the apparatus can provide spectroscopic data (which varies in response to different wavelengths) of the deposit (particle), it can be used to identify the composition of surface contaminants. Finally, since the surface to be inspected moves in accordance with a predefined motion relative to the scanning beam, erroneous detection of particles suspended in the air may be avoided.
One advantage of the invention is that high resolution may be obtained. In prior systems if two particles fell within one pixel no discrimination could be obtained. The present invention allows this situation to be distinguished and categorized by size and location.
These and other objects of this invention will become apparent to one skilled in the art from the detailed description and the accompanying drawings illustrating features of this invention by way of example.